Adhesive material

ABSTRACT

The adhesive material contains a radical-polymerizable compound, a curing agent, and a thermoplastic resin; gives a negative result of the Ames test; and has a PII, or skin irritation, value of 2 or less. In particular, all the starting ingredients should preferably give a negative result of the Ames test and have a PII, or skin irritation, value of 2 or less.

This application is a reissue continuation of Ser. No. 11/504,092 filedAug. 15, 2006 abandoned, which is a reissue of Ser. No. 09/799,634,filed Mar. 7,2001, now U.S. Pat. No. 6,777,478.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an adhesive material used for mountingbare IC chips and other electronic components on circuit boards.

2. Description of the Related Art

Insulating adhesives or anisotropically electroconductive adhesives inthe form of pastes, liquids, or films are widely used as adhesivematerials for the mounting of bare IC chips and other electroniccomponents on circuit boards.

Starting materials for manufacturing these adhesives are selected withconsideration for their purchase price, ease of fabrication, connectionreliability, storage stability, and the like. Typical examples ofstarting materials selected with consideration for such factors includeradical-polymerizable compounds such as bisphenol A-type epoxy resins,curing agents such as imidazole-based latent curing agents andamine-based curing agents, and thermoplastic resins such as phenoxyresins and urethane-based plastic resins. Acute biological toxicity(LD50), flammability, or the like is commonly taken into account inorder to ensure that these starting materials remain safe.

Adverse biological effects are not limited to the acute toxicity causedby a one-time oral ingestion (inhalation) of a large amount of startingmaterials and include the poisoning symptoms, malignant tumors, andteratogenic developments brought about by long-term oral ingestion(inhalation) of minute amounts of starting materials, as well as theintense allergic reactions or the like brought about by skin contact.Despite all these, insufficient attention has so far been paid toeradicating these problems with respect to conventional adhesivematerials, making it difficult to conclude that the social need forsafety has been properly satisfied. In addition, environmental hormones(endocrine disruptors) that cause reproductive irregularities in livingorganisms (including humans) have recently become a cause for concern,but no arrangements have so far been made for manufacturing adhesivematerials with full consideration for these effects.

SUMMARY OF THE INVENTION

An object of the present invention, which is aimed at addressing theabove-described problems of prior art, is to provide an adhesivematerial particularly suited to mounting electronic components oncircuit boards, wherein this adhesive material is highly safebiologically despite containing a radical-polymerizable compound, curingagent, and thermoplastic resin.

The inventors perfected the present invention upon discovering that anadhesive material that gives a negative result of the Ames test and hasa PII (Primary Irritation Index), or skin irritation, value of 2 or lessis extremely safe biologically.

Specifically, the present invention provides an adhesive materialsuitable for use during the mounting of electronic components on circuitboards, wherein this adhesive material comprises a radical-polymerizablecompound, a curing agent, and a thermoplastic resin; gives a negativeresult of the Ames test; and has a PII, or skin irritation, value of 2or less.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The adhesive material of the present invention is particularly suited tothe mounting of electronic components on circuit boards, and ischaracterized by comprising a radical-polymerizable compound, a curingagent, and a thermoplastic resin; giving a negative result of the Amestest; and having a PII, or skin irritation, value of 2 or less.

The Ames test referred to herein is also known as a mutagenicity test,and is one of tests in which bacteria are used to screen chemicalsubstances for carcinogenicity or mutagenicity. Specifically, it is atest in which a mutant strain of Salmonella or Salmonella typhimuriumthat lacks the ability to biosynthesize histidine is used to detectreverse mutation to produce a strain not requiring histidine (JIS K 36002408, JIS K 3610 1605). This mutation is the result of DNA damage, andcarcinogenicity is believed to become more pronounced with increasedease of mutation. Consequently, an adhesive material having highbiological safety must give a negative result of the Ames test.

The PII, or skin irritation, value is a skin toxicity index measured inaccordance with the method defined by the Consumer Product SafetyCommission of the USA (Draize Method; The Consumer Product SafelyCommission of the USA, the Code of Federal Regulations, Title 16,Section 1500.41). A lower index corresponds to less pronounced skinirritation. In the present invention, the PII value of an adhesivematerial must be 2 or less. A material with a PII value greater than 2causes discomfort in most people, and is therefore unsuitable.

The adhesive material of the present invention is such that the adhesivematerial in itself gives a negative result of the Ames test and has aPII value of 2 or less, as described above. As long as these conditionsare met, some of the starting materials used in the preparation of theadhesive material may be mutagenic or have a PII value that is greaterthan 2. It is, however, preferable for all the starting materials usedto give negative results of the Ames tests and to have PII values of 2or less in order to yield an even safer adhesive material.

All the starting materials contained in the adhesive material of thepresent invention should preferably be non-endocrine-disruptors,particularly in order to prevent reproductive cells from being adverselyaffected. In this case, passive selection methods dispensing withmaterials known or believed to act as endocrine disruptors can be usedas concrete and practical techniques for selectingnon-endocrine-disruptors for the starting materials ((1) T. Colborn, C.Clement, “Chemically Induced Alterations in Sexual and FunctionalDevelopment: The Wildlife/Human Connection,” Princeton, N.J.: PrincetonScientific Publishing (1992); (2) T. Colborn, F. vom Saal, A. M. Soto,Environmental Health Perspective, 101, 5 (1993); (3) G. Lyons,“Phthalates in the Environment,” World Wildlife UK (1995); (4) “Ministryof Agriculture, Fisheries and Food, Effects of Trace Organics on Fish,Phase II,” Foundation for Water Research UK (1995).).

The radical-polymerizable compound used in the adhesive material of thepresent invention functions as the adhesive component of the adhesivematerial. Examples of such materials include unsaturated polyesters,acrylic acid esters, and methacrylic acid esters. Two or more of thesemay be used jointly.

Non-vinyl-based oligomers synthesized from glycols, and an unsaturateddibasic acids (maleic anhydride, fumaric acid, and the like) can becited as examples of such unsaturated polyesters. Of these, diallylphthalate oligomers capable of improving adhesion to ITO electrodes arepreferred.

The acrylic acid esters may be acryl-based monomers or oligomers havingone or more, and preferably up to four, acryloyl groups in theirmolecular structures. The methacrylic acid esters may be methacryl-basedmonomers or oligomers having one or more, and preferably up to four,methacryloyl groups in their molecular structures. Examples of preferredcompounds include 1,3-butanediol methacrylate, neopentyl glycoldimethacrylate, 2-hydroxypropyl methacrylate, hydroxyethyl methacrylate,alicyclic modified neopentyl glycol acrylate, phenol ethyleneoxide-modified acrylate, polypropylene glycol diacrylate, andditrimethylol propane tetraacrylate. All these have a PII value of 0.0.

The isocyanate-derived urethane acrylate and the epoxyacrylate-basedpolymers (derived from bisphenol A or bisphenol F) commonly used asradical-polymerizable compounds should be dispensed with in the presentinvention because of their mutagenicity.

A curing agent produces dissociated radicals under the action of heat orlight. An organic peroxide is preferred as such a material. Aperoxydicarbonate is particularly preferred. Commonly usedimidazole-based latent curing agents are mutagenic and should not beused in the present invention.

A thermoplastic resin primarily functions as the film-forming componentof an adhesive material. Saturated polyester resins and polyvinylacetate polyvinyl acetal can be cited as examples of such materials.

The starting materials of the inventive adhesive material shouldpreferably contain electroconductive particles. The adhesive material ofthe present invention can therefore be used as an anisotropicallyelectroconductive adhesive. Gold particles can be cited as preferredelectroconductive particles. Common nickel particles cause skinirritation and should not be used in the present invention.

Various non-mutagenic additives whose PII values are 2 or less, such ascoupling agents and compounds containing glycidyl groups, may also beused as needed to obtain the adhesive material of the present invention.

The adhesive material of the present invention can be manufactured usinga common technique to uniformly mix such radical-polymerizablecompounds, curing agents, and thermoplastic resins.

The adhesive material of the present invention can be used in a varietyof applications involving TCPs (Tape Carrier Packages), FPCs, PWBs,glass, and plastic wiring materials, and is particularly suitable foruse in the mounting of bare IC chips on circuit boards. Specifically, abare IC chip can be connected to a wiring circuit board by a method inwhich the adhesive material of the present invention is fed onto thecircuit board on which a bare IC chip is to be mounted, the bare IC chipis positioned thereon, and the assembly is heated under pressure.

EXAMPLES

The present invention will now be described in detail through examples.

Examples 1-3, Comparison Examples 1-2

Adhesive materials whose compositions are shown in Tables 1 and 2 wereused, and anisotropically electroconductive films 30 μm in thicknesswere fabricated by a common technique.

Whereas the components used in Examples 1-3 were not confirmed to haveany endocrine disruptive action, the radical-polymerizable compounds,curing agents, and thermoplastic resins used in Comparison Example 1were confirmed to have a endocrine disruptive action, and similarly theradical-polymerizable compounds and thermoplastic resins used inComparison Example 2 were not confirmed to have a endocrine disruptiveaction.

TABLE 1 Muta- PH Examples (wt. pts) Component genicity value 1 2 3Radical-polymerizable compound Polypropylene glycol diacrylate⁺¹negative 0.0 35 20 15 Neopentyl glycol dimethacrylate⁺² negative 0.0 —15 10 Phenol EO-modified acrylate⁺³ negative 0.0 — — 10 Curing agentPeroxydicarbonate⁺⁴ negative —  5  5  5 Thermoplastic resin Polyvinylacetal⁺⁵ negative — 15 15 15 Saturated polyester⁺⁶ negative — 35 35 35Electroconductive particles Gold particles (4.5 μm) negative — 10 10 10Table 1 remarks ⁺¹M-225, made by Toagosei ⁺²NK Ester NPG, made byShin-Nakamura Chemical ⁺³M-102, made by Toagosei ⁺⁴Percure TCP, made byNippon Oil & Fats ⁺⁵Eslec BL-1, made by Sekisui Chemical ⁺⁶EritelUE3200, made by Unitika

TABLE 2 (wt. pts) Muta- PH Comparison Ex. Component genicity value 1 2Radical-polymerizable compound Bisphenol A epoxy resin⁺⁷ positive — 25 —Bisphenol A modified epoxy- (positive) 1.7 — 20 acrylate⁺⁸ Phenoxyethylacrylate⁺⁹ — 1.5 — 15 Curing agent Imidazole-based latent curingpositive — 35 — agent⁺¹⁰ Peroxydicarbonate⁺⁴ negative — —  5Thermoplastic resin Phenoxy resin⁺¹¹ (positive) — 30 50Electroconductive particles Ni particles⁺¹² (4.5 μm) (positive) — 10 10Table 2 remarks ⁺⁴Percure TCP, made by Nippon Oil & Fats ⁺⁷EP-828, madeby Yuka Shell ⁺⁸Ebecryl 150, made by UCB ⁺⁹SR-339, Satoma ⁺¹⁰NovacureHX3941HP, made by Asahi Chemical ⁺¹¹PKHH, made by Tomoe Kogyo⁺¹²Ni-J-20, made by Fukuda Kinzokuhaku Kogyo

Evaluation

The anisotropically electroconductive films fabricated according to theexamples and comparison examples were subjected to the Ames test todetermine mutagenicity and the Draize test to determine skin irritationin the manner described below. Measurements were also conducted in orderto determine the conduction resistance and adhesive strength immediatelyafter connection, and the conduction resistance and adhesive strengthfollowing aging.

Ames Test

This test was performed using direct reverse mutation of a mutant strain(his−) of Salmonella requiring histidine to a non-requiring strain(his+) under the action of a test substance. A specific test methodinvolved preparing histidine-free media to which a metabolically activesubstance (S9) bad been added, and histidine-free media devoid of themetabolically active substance (S9). The test substance was admixed intothese media, a strain (his−) requiring histidine was cultured, and thebacterial colony count of a medium containing the test substance wascompared with the bacterial colony count of a medium devoid of thesubstance. If the first value was greater than the second, the testresult was concluded to be positive, otherwise the result was concludedto be negative. The results thus obtained are shown in Table 3.

Draize Test

Hair was removed with the aid of an electric hair clipper from thelumbar regions of six healthy rabbits (body weight: 2.0-4.0 kg) toexpose the skin over an area of about 10 cm². The skin on the right sideof the vertebra was incised with a surgical knife over an area of 2.5cm², creating a lesion. The skin on the left side of the vertebra wasleft intact, and pads obtained by applying 0.5 mL samples were affixedat two locations on the right and left of the vertebra. The pads werepeeled off 24 and 72 hours following affixation, redness and swellingwere visually observed, the results were graded using a five-pointsystem (levels 0 to 4), a gross mean was obtained using the averagevalues for each rabbit, and the gross mean value was termed the PIIvalue. The results thus obtained are shown in Table 3.

Conduction Resistance and Adhesive Strength Immediately After Connection

An anisotropically electroconductive film slitted to a width of 2 mm wassandwiched between 200-μm pitch TAB and a 200-μm pitch PWB, the assemblywas thermocompression-bonded at 160° C. and 2.94 MPa (30 kgf/cm²) for 20seconds, and the conduction resistance immediately after compressionbonding and the adhesive strength resulting from 90° peeling weremeasured using a tensile testing machine (RTC-1210, manufactured byOrientec). The results are shown in Table 3.

Conduction Resistance and Adhesive Strength Following Aging

An anisotropically electroconductive film slitted to a width of 2 mm wassandwiched between 200-μm pitch TAB and a 200-μm pitch PWB, the assemblywas thermocompression-bonded at 160° C. and 2.94 MPa (30 kgf/cm²) for 20seconds, and the conduction resistance resulting from allowing theassembly to stand for 1000 hours in an 85° C.-85% RH environment wasmeasured together with the adhesive strength resulting from 90° peelingby means of a tensile testing machine (RTC-1210, manufactured byOrientec). The results are shown in Table 3.

TABLE 3 Examples Comparison Ex. 1 2 3 1 2 Mutagenicity negative negativenegative positive positive PH value 0.0 0.0 0.0 5.5 8.3 Conductionresistance (Ω) Immediately after 0.032 0.032 0.032 0.032 0.032thermocompression bonding Following aging 0.035 0.035 0.035 0.035 0.035Adhesive strength (gf/cm) Immediately after 740 850 900 900 700thermocompression bonding Following aging 700 730 850 850 700

The results in Table 3 indicate that the adhesive materials of Examples1-3 have the same conduction reliability and adhesive strength as theconventional adhesive materials (Comparison Examples 1 and 2), and usingonly safe starting ingredients for these adhesive materials ultimatelyallows these materials to exhibit no discernible mutagenicity, to have aPII value of 0.0, and to remain highly safe.

The adhesive material of the present invention has the same conductionreliability and adhesive strength as conventional adhesive materials,exhibits no discernible mutagenicity, has a PII value of 2 or less, andis an extremely safe adhesive material.

The entire disclosure of the specification, summary and claims ofJapanese Patent Application No. 2000-077219 is hereby incorporated byreference.

1. An adhesive material containing a radical-polymerizable compound, acuring agent, a thermoplastic resin, and electroconductive particles,wherein the radical-polymerizable compound is at least one compoundselected from the group consisting of diallyl phthalate oligomer,1,3-butanediol methacrylate, neopentyl glycol dimethacrylate,2-hydroxpropyl methacrylate 2-hydroxypropyl methacrylate, hyroxyethylmethacrylate, alicyclic modified neopentyl glycol acrylate, phenolethylene oxide-modified acrylate, polypropylene glycol diacrylate, andditrimethylol propane tetraacrylate, and the thermoplastic resin is asaturated polyester resin or polyvinyl acetate, and wherein saidadhesive material gives a negative result of the Ames test and having aPII, or skin irritation, value of 2 or less.
 2. An adhesive material asdefined in claim 1, wherein all the starting ingredients of the adhesivematerial give a negative result of the Ames test and have a PII, or skinirritation, value of 2 or less.
 3. An adhesive material as defined inclaim 1, wherein all the starting ingredients of the adhesive materialare non-endocrine-disruptors.
 4. A mounting method for connecting a bareIC chip to a wiring circuit board comprising feeding an adhesivematerial as defined in claim 1 onto a circuit board on which a bare ICchip is to be mounted, positioning the bare IC chip thereon, and heatingthe assembly under pressure.
 5. An adhesive material as defined in claim1 wherein the electroconductive particles are gold.
 6. An adhesivematerial as defined in claim 1 wherein the radical polymerizablecompoundat compound is at least one compound selected from the groupconsisting of neopentyl glycol dimethacrylate, phenol ethyleneoxide-modified acrylate, and polypropylene glycol diacrylate.